Fuel is prepared to have a certain Reid Vapor Pressure (RVP) before being delivered to an underground storage tank at a service station for later dispensing into a vehicle. RVP is measure of a fuel's volatility at a certain temperature and is a measurement of the rate at which fuel evaporates and emits volatile organic chemicals (VOCs), namely hydrocarbons (HCs). RVP is measured by measuring the pressure of fuel vapor at a temperature of 100 degrees Fahrenheit. The higher the RVP, the greater the tendency of the fuel to vaporize or evaporate. The RVP of fuel can be lowered by reducing the amount of a volatile liquid's most volatile components, such as butane in gasoline fuel for example.
In a service station environment, fuel having a higher RVP, for example 14 pounds per square inch (Psi), is typically delivered during the winter months, whereas fuel having a lower RVP, for example 7 Psi, is typically delivered during the summer months. The reason that it is desirable to deliver fuel to a service station having a lower RVP during the summer months is that this can offset the effect of higher summer temperatures upon the volatility of the fuel, which in turn lowers emissions of VOCs. Emissions of VOCs cause product of ground level ozone and increased exhaust emissions from vehicles. During the winter months, it is desirable to provide fuel having a higher RVP, which ignites easier in colder temperatures.
In service stations employing Stage II vapor recovery systems, the vapor emanating from the vehicle tank during refueling is recovered and is returned to the underground storage tank. During the summer months, the vapor recovered and collected from the vehicle tank has a higher temperature than the underground storage tank. Therefore, the collected vapor shrinks in volume in the underground storage tank due to this temperature differential. It is also less likely for summer fuel, having a lower RVP, to evaporate in the underground storage tank and create vapor growth and therefore volume increase.
During the winter months, the vapor emanating from the vehicle tank collected and returned to the underground storage tank is lower in temperature than the underground storage tank. As a result of this temperature differential, the recovered vapor from the vehicle expands in volume when it enters the underground storage tank. Additionally, the vapor returned to the underground storage tank reacts with the higher RVP fuel in the underground storage tank and vapor growth occurs due to the high volatility of the fuel. This further increases vapor growth in the underground storage tank. If the pressure in the underground storage tank reaches a certain threshold level, a vent to atmosphere is opened to release this excess pressure so that the underground storage tank is not over-pressurized. This release of excess pressure causes vapors or VOCs to be released into the atmosphere thereby causing harm to the environment.
Therefore, a need exists to provide a system and method to keep vapors collected from a vehicle during refueling and resident in the underground storage tank from expanding in the underground storage tank to keep pressure from increasing and releasing VOCs to atmosphere.